Project Summary/Abstract Alternative splicing is a central mechanism to diversify genetic information on the post-transcriptional level. Advances in sequencing technologies revealed shifts in alternative splicing patterns as key features in a variety of biologically relevant systems including embryo development, the adaptive immune response and cancer progression. A recent RNAseq study demonstrated that alternative splicing patterns for thousands of transcripts are altered in macrophages infected with Listeria. While proteins and mechanisms involved are not established, a protective cellular response to limit intracellular replication may be a consequence. The central goal of this proposal is to use this infection model system to gain insights into dynamics of non-coding RNAs and mechanisms of alternative splicing on a single cell level. Intriguingly, it was independently discovered that spliceosome components are transiently sequestered in cytosolic RNA-protein granules called U-bodies during Listeria infection, suggesting that spatiotemporal sequestration may contribute to alternative splicing regulation. Infection with Listeria and formation of U-bodies are highly heterogeneous both in space and time and ideally must be assessed on a single-cell basis. Fluorescence microscopy offers the possibility for long-term visualization of tagged proteins and fluorescently labeled pathogens, but robust tools to visualize cellular RNAs are limiting. To enable visualization of non-coding RNAs, a versatile tool to fluorescently label RNA in live cells will be developed (Aim 1). This tool will then be utilized to quantify spatiotemporal dynamics of U-bodies and simultaneously monitor Listeria replication (Aim 2). Contributions of spliceosome components will be dissected by monitoring RNA dynamics and Listeria replication as spliceosome components will be manipulated experimentally. Lastly, a time resolved quantitative mass spectrometry approach will be used to identify protein candidates that regulate re-shaping of the alternative splicing landscape (Aim 3). These candidate factors will be further investigated by knockdown and assessing consequences for U-body dynamics and intracellular bacterial replication in the microscopy assay. Together, this study will serve as a unique model system to unravel alternative splicing regulation on a single cell level in a physiologically relevant model system using fluorescence microscopy.